How i can let users add notes to my posts

There are number of possibilities here:

1- Giving users the ability to edit posts and add notes. This is not something you want to do. As it might create many issues for you. However, you can use a Wiki plugin if you want to pursue this option.

2- Enabling notes through comments. This is a better option. You can handle user comments and ask whether he wants to add it as a comment or a note to be added to the post. You need to modify WordPress code and develop your own code as well.

3- Using footnotes plugin as notes system. You can use any of the plugins available for footnotes and possibly customize it yo use it a notes system. This plugin seems to be good. It displays the footnote inside the text. You can modify the plugin code to display a note instead of a footnote.